<p>This study examines the thermodynamic behaviour of coprecipitation reaction for Ni_{1-<i>x</i>-<i>y</i>}Co_<i>x</i>Mn_<i>y</i>(OH)₂ precursors that is necessary for preparation of cathode materials, LiNi_{1-<i>x</i>-<i>y</i>}Co_<i>x</i>Mn_<i>y</i>O₂, of Li-ion batteries under high-ionic-strength process solutions. To capture non-ideal solution behavior while maintaining practical parameter availability, a hybrid activity framework is proposed. Pitzer interactions are applied to dominant background ions including Na⁺, NH₄⁺, and SO₄^2-, while Davies and extended Debye–Hückel corrections are employed for metal-ammine and metal-hydroxo complexes. Using a consistent activity-based formulation, the total solubilities of Ni²⁺, Co²⁺, and Mn²⁺ are calculated and visualized exclusively within the precipitation-relevant regime. Under the industrially relevant pH range (9 - 12), optimum pH ranges are identified based on inter-metal disparity and precipitation margin as a function of composition of precursors and total ammonia concentration. Compared with concentration-only or Davies-only approaches, the hybrid framework yields a more conservative and practically relevant operating window for pH and ammonia control. Although experimental calibration is not included, the framework provides an activity-consistent solubility map suitable for extension with refined equilibrium data.</p>